The present invention concerns a method for heating a roll, in particular of a press roll or calender roll in a paper or board machine, said roll comprising a hollow, tubular roll mantle.
The present invention also concerns a heatable roll intended for carrying out the method, in particular a press roll or calender roll in a paper or board machine, comprising a hollow, tubular roll mantle, which is provided with heating means for heating of the roll mantle.
The rolls are used in paper machines and in paper finishing equipment for performing various functions at a number of locations. The most common places for the use of such hot rolls are in calenders and presses, as well as in supercalenders. The commonest solution used in the art for heating of a roll is to introduce the heat into the roll by means of a suitable heat transfer medium, such as water or, if a roll operating at an even higher temperature than is possible using water is required, by means of oil. A number of different methods and solutions for the heating of rolls are known in the art. The oldest prior-art solution is one in which a through hole is drilled into the center of a massive roll, and the heat transfer medium is circulated through this hole in order to heat its surface. However, it is an essential drawback of such a solution that the distance from the heat transfer medium to the face of the roll is very large, thus efficient and economical heating cannot be achieved by means to this solution, since the heat must be conducted through the entire material of the roll. A second, more advanced solution is one in which the roll comprises a non-revolving cylindrical body or axle on which a revolving roll mantle is mounted. In this solution, the heat transfer medium, which may be water or oil, is flows through the space between the non-revolving body and the roll mantle, to heat the roll mantle. This method reduces the distance through which the heat must be conducted to effectively heat the roll surface. One solution of this type is described, e.g., in EP Patent Application No. 0,188,238. However, it is an essential drawback of such a solution that a large quantity of liquid must be dealt with. This is because the free space for the flow of the liquid in the roll is large, and this fluid must be circulated in order to maintain operating temperature, Further, because of the great volume of heat-transfer liquid, the method of incorporating a roll of this type characteristically has a very high power consumption. Other heatable rolls are known, which heat the roll with a similar construction, e.g., in the FI Patent Application Nos. 840,458, 862,189 and 864,020 as well as EP Patent Application No. 0,158,220. A further solution for providing a heated roll is one in which a number of through-bores are formed axially oriented in the roll in which the heat-transfer medium is circulated. One such solution is described in prior art, e.g., in the Canadian Patent No. 1,223,763. All of the prior-art heatable rolls described above involve the drawback already stated above, i.e. that large quantities of liquid must be dealt with due to the necessity of fluid circulation and heating of the fluid outside the roll. It is a further drawback that the heat-transfer medium must first be heated in some suitable way, e.g. by means of electric power, prior to being pumped through the bores of the roll. The power consumption required to operate such rolls is remarkably high.
The prior art also discloses variable crown rolls which are heatable. Examples of such solutions are described in U.S. Pat. Nos. 4,282,639 and 4,679,287. In U.S. Pat. No. 4,282,639, the roll is heated by an arrangement of spraying devices, fitted in the interior of the roll, into which devices a heated heat-transfer medium is fed and sprayed onto the inner face of the roll mantle, to heat the roll mantle. On the other hand, in the U.S. Pat. No. 4,679,287, the heating of the roll is arranged so that the heating medium is passed directly into the interior roll, into the space between the stationary roll axle and the roll mantle for the purpose of heating of the roll mantle. With regard to heating the roll mantle, such variable crown rolls also involve the same drawbacks as were already discussed earlier, including dealing with the large quantities of liquid, the problems related to the heating of the heat-transfer medium, as well as the high power consumption of the apparatus.
It is an object of the invention is to provide a method for heating a heatable roll wherein the power consumption of the roll is reduced while at the same time achieving a more uniform heating of the roll mantle. In view of achieving these objectives, the method of the present invention is characterized in that the roll is heated by electric heaters, e.g. by passing an electric current through electrical resistors, which are contained in the body material of the roll mantle.
If is another object of the present invention that the heating means for the roll comprises ducts, conduits or the equivalent, which have been formed into the body material of the roll mantle, extending from one end of the roll to the other, with electric heaters fitted in the ducts and connected to an electrical power source in order to heat the electric heaters inside the body material of the roll mantle.
It is a further object of the present invention to provide a variable crown roll having electric heaters formed material of the roll mantle.
It is a still further object of the present invention to provide an electrically heatable roll in which the heating elements are divided into axially spaced zones, wherein the heating of each zone is controllable.
By means of the present invention, a number of remarkable advantages are obtained, as compared with the prior art. In the method in accordance with the present invention, the quantity of oil or other heat-transfer medium required is small and, in connection with variable crown rolls, the medium can be maintained at a comparatively low temperature. Since, in the present invention, the heat is introduced at a position very close to the outer surface of the roll mantle, the thermal strains can be reduced and maintained at low levels, and the temperature profile of the roll can be kept substantially more uniform than in rolls heated by a means of a circulating liquid. Large temperature gradients which may occur in the roll mantle in the prior art methods are eliminated. In connection with a roll in accordance with the present invention, in heating the surface it is unnecessary to use various complicating devices placed external to the roll such as a liquid heater and circulating pump. Further, the heating efficiency can be made optimal, almost 100 percent, because the heat dissipation is almost entirely through the roll face, and other sources of inefficiency are almost entirely eliminated.
A roll in accordance with the invention is also inexpensive to manufacture. The roll can be constructed out of commercially available components. In addition to allowing maintenance of a uniform temperature profile, the present invention facilitates the limited warming of the end areas of the roll in the desired way through zoned heating. Further advantages and characteristic features of the invention are apparent from the following detailed description of the invention.